Method of making moisture resistant paperboard

ABSTRACT

IN A LIQUID CONTAINER FORMED OF POLYETHYLENE COATED PAPERBOARD, WICKING THROUGH EXPOSED RAW EDGES ON THE INTERIOR OF THE CARTON IS SUBSTANTIALLY REDUCED BY TREATING THE PAPERBOARD WITH SMALL QUNATITIES OF A MOISTURE RESISTANT MATERIAL, SUCH AS WAX, PRIOR TO THE APPLICATION OF THE POLYETHYLENE COATING. THE WAX IS APPLIED TO THE PAPERBOARD IN MOLTEN FORM IN THE RANGE OF APPROXIMATELY 1 TO 7 PERCENT BY WEIGHT OF THE UNCOATED PAPAERBOARD AND DRIVEN EVENLY AND UNIFORMLY THROUGHOUT THE BOARD BY THE SUBSEQUENT APPLICATION OF HEAT.

J. P. DWYRE 3,561,994

METHOD OF MAKING MOISTURE RESISTANT PAPERBOARD Feb. 9,1971

Filed May 5. 1967 INViliN'l'UR. James P. Dwyre ISY ATTORNEY United States Patent 3,561,994 METHOD OF MAKING MOISTURE RESISTANT PAPERBOARD James P. Dwyre, Stamford, Conn., assignor to Westvaco Corporation, New York, N.Y., a corporation of Delaware Filed May 3, 1967, Ser. No. 635,829 Int. Cl. B65d 5/56, D21h 1/36 US. Cl. 117-46 5 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Field of the invention The prevention of wicking through an uncoated raw edge of a paperboard carton.

Description of the prior art Coated paperboard is used extensively today for the packaging of a variety of products, such as liquids and frozen foods. Milk, for example, is primarily packaged in a carton formed of paperboard coated on both sides with polyethylene; as in the familiar gable top carton. This same type of carton is also used extensively for the packaging of the ade drinks, that is, lemonade, orangeade and the like.

Since the paperboard stock material from which the carton blanks are formed is coated before the paperboard is cut into blanks, the cut edges will be uncoated and susceptible to penetration by liquids. In the erection of most paperboard containers, the blanks are first formed into an open ended tube by overlapping one edge of the blank over the opposite edge; thereby placing at least on no coated or raw edge on the interior of the package. When the carton is subsequently filled with a liquid, the phenomenon of wicking will occur; that is, the penetration or absorption of liquids into or by the raw, uncoated edge. While this has not proven too serious a problem in the packaging of products such as milk, the ades present a different situation because of their much lower solids content and longer shelf life. Thus, it often occurs that the ades penetrate into the paperboard through the raw or uncoated edge of the carton to the extent that the carton walls become seriously weakened.

Pat. No. 3,198,416 discusses this problem and discloses one approach to its solution. However, this solution, and all others of which applicant is aware, have in common the application of a sealant of some type to the raw edge of the carton blank after the blank has been severed from the web. Thus, in all prior art methods of which applicant is aware, the approach is to treat the individual cartons or carton blanks rather than the stock material from which the cartons are formed. Obviously, if the continuous web of carton stock material is treated, rather than the individual cartons or carton balnks, the efficiency of carton production will be greatly enhanced.

3,561,994 Patented Feb. 9, 1971 SUMMARY In accordance with the present invention, wicking is substantially reduced in paperboard containers by applying small amounts of a moisture resistant material, such as wax or wax like compositions, to the paperboard web prior to the subsequent applications of a moisture proof coating thereto and driving the material evenly and uniformly into the web by the application of heat.

While wax impregnation of paperboard has been known as a means of increasing its moisture resistance, it had previously been thought that the wax had to be applied at fairly high concentrations in order to obtain appreciable moisture resistance. At these concentrations, however, the bonding of a layer of polyethylene or the like to the paperboard is difllcult, if not impossible, because of the lack of wax free fibers in the paperboard surface to form a fiber bond with the polyethylene.

Applicant has discovered, however, that if the wax content of the paperboard is maintained below 7 percent by weight of the uncoated paperboard, suflicient wax free fibers remain on the surface of a board to form a good bond with the subsequently applied coating. Additionally, it has also been discovered that wax concentrations above 1 percent by weight, applied evenly and uniformly throughout a paperboard web, will substantially reduce the phenomenon of raw edge wicking in the finished carton.

It is therefore, an object of the present invention to provide a liquid proof, paperboard carton in which raw edge wicking is negligible.-

It is a further object of the present invention to produce a carton of the type described above by a method which does not require treatment of the raw edges of the individual cartons or carton blanks.

Thus, in accordance with the present invention wicking resistance of the paperboard cartons is greatly improved by the continuous treatment of the paperboard web from which the cartons are subsequently formed.

These and other objects and advantages of the present invention will become more readily apparent from the following detailed description wherein:

FIG. 1 is a perspective view, with portions broken away, of a typical paperboard container;

FIG. 2 is a cross-sectional view taken on line 22 of FIG. 1; and

FIG. 3 is a somewhat schematic representation of a method of producing paperboard carton stock material according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 of the drawings, an example is shown of a container in which moisture resistant paperboard according to the present invention may be used to advantage. The container, as seen in FIG. 1, may be of the familiar gable top type, which comprises an upright side wall portion including side wall panels 2-5, a bottom closure 6 and a top closure 7. As shown in the example of FIG. 1, and typical of many paperboard cartons, a seam 8, which is formed by overlapping one edge of the carton blank and securing it to its opposite edge, runs longitudinally of the carton; disposing an uncoated edge to the interior of the carton. While this is a conventional and a convenient way of forming paperboard cartons, it is also a possible source of trouble; particularly in the packaging of some materials.

Thus, as seen in FIG. 2, the paperboard of which the carton is formed will generally be coated on one or both sides with a layer of thermoplastic, resinous material, such as polyethylene or the like. The coatings as at 10 and 11, therefore, protect a major portion of the paperboard of the carton from penetration by the products contained therein. However, when the paperboard carton blanks are formed, they are stamped from a web which has been previously coated, leaving a raw uncoated edge, as at 12, which, when the carton is erected, affords a path for the penetration of moisture into the walls of the carton.

While this phenomenon, which is known as wicking does not present too serious a problem in the packaging of many products, in the packaging of drinks, such as lemonade, orangeade, etc., which have a low solids content and a relatively long shelf life, wicking may eventually cause a serious weakening of the carton walls and eventual deterioration.

Applicant has discovered, however, that if a relatively small amount of a moisture resistant material is distributed internally of the paperboard prior to coating with a thermoplastic material, wicking through the raw or uncoated edge of the carton may be drastically reduced. Referring to FIG. 3, one method of accomplishing this will be described in detail.

As seen in FIG. 3, a web 13 of paperboard, suitable for carton manufacture, is unwound from a reel 14 and passed beneath a pressure roll 15 which presses the web against an applicator roll 16. A pick up roller 17, rotating in a bath of moisture resistant material 18, carried by a vat 19, transfers a layer of the material 18 to the roll 16 where it is applied to the web.

The material 18 may be any of the well known waxes or wax compositions such as wax compounds consisting mainly of petroleum waxes, of which the parafiin waxes are an example. In a specific example a wax known as Atlantic No. 173 was found to give suitable results. This wax, which is produced by The Atlantic Refining Company, is a fully refined wax having an American melting point range of 133-136 F. (A.S.T.M. D-87 melting point of 131.5), a Saybolt color of +30 (A.S.T.M. D-156), penetration at 100 F. of 45 (A.S.T.M. D-l321), viscosity at 160 F. of 5.30 cs. (A.S.T.M D-445) and oil content of 0.3 (A.S.T.M. D721).

From the roll 15 the web is trained past a burner 20, which serves the dual purpose of burning off any loose fibers on the opposite side of the web and heating the web to cause the material 18 to penetrate into the interior of the paperboard material. From the burner 20 the web is trained around a preheat roll 21 which further serves to drive the material 18 evenly and uniformly throughout the paperboard and also serves to prepare the paperboard for the coating applied at extruder 23.

The extruder 22 may be of the conventional design which deposits a thin film of a thermoplastic, resinous material on one surface of the web 13 as it is drawn past the extruder. The material applied to the web surface may be any of the well-known, thermoplastic resinous materials which are substantially moisture proof, non-toxic, and flexible, such as polyethylene.

Coming from the extruder, the web is passed around a chill rod 23 and thence, around various tension and draw rolls past a second burner 24, which serves to remove any loose or protruding fibers from the opposite side of the web. The web then passes to a second extruder 25 where a second layer of thermoplastic resinous material is applied to the opposite side of the web. The web is then trained around a second chill roll 26, thence, around tension and draw rolls to a conventional treating device 27, which renders the coating applied to one side of the web more receptive to inks for printing operations. From this point the web is passed around another series of draw and tension rolls to a rewind stand 28 where the web is rolled up in a form suitable for subsequent carton processing.

-It should be noted that in the application of the material 18 to the web 13, the amount of moisture resistant material applied to the web must be carefully controlled between certain predetermined limits. Thus, it has been found that if the material applied to the web exceeds approximately 7 percent by weight of the uncoated paperboard, an unacceptable bond between the paperboard and the coatings of thermoplastic resinous material will result. On the other hand, if the concentrations of the moisture resistant material fall below a certain amount, the wicking resistance characteristics of the present invention will not be obtained. Thus, it has been found that if the amount of material 18 distributed throughout the web is less than approximately 1 percent by weight of the uncoated paperboard, there will be no appreciable gain in wicking resistance.

It will be seen, therefore, that in order to obtain the benefits of the present invention in a paperboard coated with a thermoplastic resinous material such as polyethylene, the amount of moisture resistant material, such as wax or wax compositions, should be distributed throughout the board in the range of approximately 1-7 percent by weight of the uncoated paperboard.

While specific examples of the moisture resistant paperboard, the method of manufacture, and an article formed therefrom have been described in detail, it will be obvious that variations thereof will occur to those skilled in the art.

What is claimed is:

1. The method of producing moisture resistant paper board in continuous web form comprising:

(a) coating a surface of said web with a waxy material applied in an amount equal to at least one percent but not exceeding seven percent by weight of said paperboard web;

(b) heating the opposite surface of said web to a temperature suficiently high to effect removal of loose fibers from said surface and cause distribution of said waxy material throughout said web in preparation for subsequent application thereto of a polyethylene coating, and

(c) coating said opposite surface with polyethylene.

2. The method of producing moisture resistant paper board comprising the steps of:

(a) applying a waxy material to an obverse surface of a continuously advancing paperboard web in an amount equal to at least one percent but not exceeding seven percent by weight of said paperboard web;

(b) heating said web from its reverse surface to cause said waxy material to penetrate into said web and to facilitate removal of any loose fibers from a said reverse surface;

(c) depositing a thin film of polyethylene onto said reverse surface of said web subsequent to said heating from said reverse surface;

(d) heating said web from its obverse surface to facilitate removal of any loose fibers therefrom; and

(e) depositing a thin film of polyethylene onto said obverse surface of said web subsequent to said heating from said obverse surface.

3. The method of producing moisture resistant paperboard according to claim 2 wherein said polyethylene is deposited onto said obverse and reverse surfaces of said web by extrusion coating.

4. The method of producing moisture resistant paperboard according to claim 2' further comprising the steps of cooling said web subsequent to each of said depositing steps.

5. The method of producing moisture resistant paperboard comprising the sequential steps of:

(a) applying waxy material to an obverse surface of a continuously advancing paperboard web in an amount equal to at least one percent but not exceeding seven percent by weight of said paperboard web;

(b) drawing the reverse surface of said web past a burner to cause said waxy material to penetrate into said web and to efiiect removal of any loose fibers from said reverse surface of said web;

(0) further heating said web to facilitate even penetration of said waxy material into said web;

(d) extrusion coating polyethylene onto said reverse surface of said Web;

(e) chilling said web;

(f) drawing the obverse surface of said Web past a burner to eifect removal of any loose fibers from said obverse surface;

(g) extrusion coating polyethylene onto said obverse surface of said web; and

(h) chilling said Web.

2,984,585 5/1961 Sherman .117--158X JOHN T. GOOLKASIAN, Primary Examiner J. C. GIL, Assistant Examiner US. Cl. X.R. 

